1. Field of the Invention
The present invention relates to a magnetic cancelling apparatus for a color cathode-ray tube wherein external magnetic fields such as the geomagnetism acting on the color cathode-ray tube are cancelled to prevent deterioration of the color purity of the screen.
2. Description of the Prior Art
FIG. 1 is a sectional side view showing the relation between electron beams and phosphors in a color cathode-ray tube (hereinafter referred to as "CRT"). In the same figure, the numeral 1 denotes a display panel of the CRT; numeral 2 denotes a phosphor coated on the back of the panel 1; numeral 3 denotes a shadow mask disposed closely behind the panel 1; numeral 4 denotes a small hole formed in the shadow mask 3; and numeral 5 denotes an electron beam which impinges upon the phosphor 2 through the small hole 4.
FIG. 2 is a perspective view showing external magnetic fields acting on a display unit provided with a CRT. In the same figure, the numeral 6 denotes a display unit; numeral 7 denotes a housing of the display unit; numeral 8 denotes a vertical magnetic field acting vertically on the display unit 6; numeral 9 denotes a horizontal magnetic field acting horizontally on the display unit 6; and numeral 10 denotes an axial magnetic field acting on the display unit 6 in the direction of the axis of the CRT.
The operation will now be described. Electron beams 5 emitted from electron guns (not shown) of, for example, red, green and blue pass through small holes 4 of the shadow mask 3 and impinge upon phosphors 2 of the respective corresponding colors. Consequently, the phosphors 2 emit light to make a predetermined color image display. Where external magnetic fields such as the geomagnetism are not exerted on the display unit 6, that is, where the vertical, horizontal and axial magnetic fields 8, 9, 10 are all zero, the electron beams are in the state of just landing wherein the center of each phosphor 2 and that of each electron beam 5 are coincident with each other, as shown in FIG. 3.
When external magnetic fields such as the geomagnetism act on the display unit 6, the paths of the electron beams 5 are bent, so that the just landing state is not maintained and there occurs mislanding, as shown in FIG. 4. More particularly, as shown in FIG. 4(A), under the influence of the vertical magnetic field 8 passing from above to below, the electron beams 5 as a whole are shifted to the left in the figure with respect to the phosphors 2 on the screen of the panel 1. Moreover, under the influence of the horizontal magnetic field 9 passing from the left to the right, the electron beams 5 as a whole are shifted downwards with respect to the phosphors 2, as shown in FIG. 4(B). Further, under the influence of the axial magnetic field 10 passing from the panel 1 side toward the electron guns, the electron beams 5 are shifted clockwise with respect to the phosphors 2 on the panel 1, as shown in FIG. 4(C). Such mislandings cause deterioration in the color purity of the screen.
As a magnetic cancelling apparatus for preventing such mislandings of the electron beams 5 relative to the phosphors 2 caused by external magnetic fields, there has heretofore been known an apparatus wherein an internal magnetic shield formed of a magnetic material is provided in the interior of a CRT. Also known is a geomagnetism cancelling apparatus which is disclosed in the Technical Reports of the Institute of Television Engineers of Japan, ED913 IPD101-16, Nov. 15, 1986. In this known geomagnetism cancelling apparatus, a cancelling coil is mounted to a CRT, and by passing an electric current in the cancelling coil there is generated a magnetic field in a direction of cancelling the geomagnetism.
In the conventional magnetic cancelling apparatus for a CRT of the above construction provided with the internal magnetic shield, it is impossible to obtain a satisfactory magnetic cancelling effect. Particularly, under the action of the axial magnetic field 10, there occurs a so-called clipping phenomenon in which, for example, an electron beam 5 which should impinge upon a green phosphor 2 impinges upon an adjacent red or blue phosphor 2. This phenomenon is apt to occur in a CRT having small gun separation. To avoid such clipping, the area, or the diameter, of each phosphor 2 is made smaller at the marginal portions than at the central portions of the screen. In this case, however, there occurs a difference in luminance between the central portions and the marginal portions of the screen, resulting in that the marginal portion becomes dark.
In the conventional geomagnetism cancelling apparatus also referred to above, when the panel 1 of the CRT is moved in a specific direction, the current in the cancelling coil is adjusted manually to obtain the best purity. Therefore, if the display unit 6 is changed its direction, the purity will be deteriorated. So at every such change it is necessary to make manual adjustment, which is inconvenient.